To Squat or Not to Squat: How Low can you Go?

Written by Ian McDonald, Crop Innovations Specialist, OMAFRA and Alex Barrie, Soil Management Engineer, OMAFRA

The impact of farm equipment causing soil compaction is primarily a function of soil moisture at the time of traffic coupled with the total weight, axle load and tire pressure of the implement. By lowering the tire pressure, we reduce the impact of soil compaction up to a point which is a function of the ability of a tire to tolerate the load and speed in an under-inflated state.

Tire pressures in bias tires cannot be reduced for many reasons including the technology of the tire, how it stays on the bead, etc. Radial tires have the ability to reduce tire pressure and keep the integrity of the tire so long as it occurs under slow speeds operating in a straight line. Reducing tire pressures and running on the road greatly reduces safety and increases fuel consumption and tire wear. Although central inflation/deflation systems (CTIS) are becoming more common, the tire industry is still adjusting to farmers demand for reduced soil compaction through reduced tire pressures during field operations. Many producers are incorporating these reduced tire pressures as a whole or with CTIS systems and accepting that they may be voiding the warranty on the tires.

When you look at a tire that has a significant bulge due to low inflation pressure, it appears to be getting wider and thus the impression is that it makes soil compaction occur over a larger area in the field. Comparing the images in Figure 1 below we see that the low pressure tire looks wider. While the side wall bulges, and may actually touch the soil, Figure 2 points out that tires that are deflated are getting longer, not wider in terms of weight carrying footprint. This is the goal since we have been causing compaction in the wheel track, to reduce compaction we don’t want to make the compacted area wider. With a longer footprint from reduced tire pressure the potential compaction region within the field is the same and the overall impact is lower because we have spread the implement weight over more square inches of area within the same track width.

Figure 1. The visual difference in tires inflated at road (left) and field (right) ratings using CTIS.

Many farmers look at the tires on the right side of Figure 1 and feel queasy. How can that work, they ask themselves? Farmers who have moved to CTIS systems or just made the decision to go with lower inflation pressures and road travel at a slower speed feel the exact opposite. To see a fully inflated sidewall on farm equipment tires makes them shudder!

Figure 2. The impact of tire technology choice and inflation pressure on footprint. (A) An over inflated (31 PSI) tire with an axle load of 24,500 lbs has a footprint length of 20 inches. (B) A tire with standard inflation (18 PSI) and an axle load of 24,500 lbs has a footprint length of 25 inches. (C) An increased flexibility or “IF” tire inflated to 14 PSI with an axle load of 24,500 lbs has a footprint length of 29.75 inches.

In Figure 2 the same sized tire (480/80R50) is inflated at different tire pressures in examples A,B and C at an axle load of 24,500 lbs. The red horizontal arrows are the same width and show that as the inflation pressure declines, the tire stays the same width. Note how the length of the footprint increases by 20% by reducing the tire inflation from 31 to 18 psi.

Image C which is a tire with increased flexibility or “IF” technology allowing further reduction in tire pressure, shows an increase in the footprint length by 33% compared to image A. This is very signficant in terms of the things we are interested in; reduce compaction, increase fuel efficiency, decrease tire wear/failure etc. It is important to recognize that these lower tire pressures may not be safe on the road, and may void your tire warranty.

A median tire pressure such as in image B of Figure 2 is sometimes considered a “trade-off air pressure”. The trade-off is a lose-lose situation. Based on the work by Matthias Stettler of Switzerland and others, it is thought that the common target weight of a maximum of 10T/axle of implement is too high. The targets should be maximum 5T/axle and tire pressures not exceeding 15psi. Trying to find a compromise gets you nothing. In the field this increases fuel use, does not significantly reduce the compaction potential and reduces equipment efficiency.

Some tire manufacturers have come up with tire charts that separate out road and field operations and what the appropriate tire pressures are, such as the chart in Figure 3. Note how the settings are based on a combination of load and speed. As an example, if a farmer has an axle weight of ~6000 lbs and wants to run on the road at 40mph but the field operation is at 6mph, then this tire is warrantied for 20psi on the road (“A”) but down to 12psi in the field (“B”). In this example that is a 40% reduction in tire pressure which will significantly reduce the possibility for soil compaction under wet soil conditions.

Figure 3. A tire chart showing the change in individual tire pressure setting as a function of weight and speed. Image Credit: Adapted from Michelin

There are a significant number of implements being purchased with bias tires. If axle weight exceeds 5T/axle on these types of implements, the preferred choice would be radial tires. Forage equipment is one of the worst culprits for commonly using bias tires. People don’t think about compaction on perennial stands of forage but it can be very problematic.

As farmers have a greater awareness of the impacts of soil compaction, they are demanding more from their equipment. We need to encourage equipment companies to develop designs that allow more rubber to be placed under these ever increasing sized implements. While the price of better or more tires or adapting CTIS systems is hard to stomach, the owner will be rewarded in the long term on many fronts.